Conventionally, radar devices, which are one kind of a target object detection device, transmits a pulse signal to a detection area and detects a target object from a reflection signal. In order to improve an S/N of the reception signal and to raise the target object detection performance, the radar device using pulse-compression processing exists for such a radar device.
In the radar device using such a modulated pulse signal, the reception signal may be saturated when amplifying the signal by an LNA (Low Noise Amplifier) if an object with a large reflective cross-sectional area exists within the detection area. If the reception signal is saturated, a peak level of the reception signal will reach the ceiling and, thus, it will be impossible to obtain an accurate target object detection result due to a drop of the S/N. Especially, in the case of the radar device using the pulse-compression processing for transmitting the pulse-shaped transmission signal containing an FM chirp signal (hereinafter, referred to as a “modulated pulse signal”), if the reception signal is saturated, a frequency component ratio of the reception signal and a frequency component ratio of the transmission signal will not be in agreement with each other. Therefore, at the time of the pulse compression, the level of the peak frequency will fall and range side lobes (side lobes on a time axis) will occur.
FIGS. 4A and 4B are views illustrating the generation of the range side lobes of the pulse-compressed signal in the conventional radar device. FIG. 4A is a time chart for showing time-axis waveforms of the transmission signal, the reception signal, and the pulse-compressed signal, and FIG. 4B is a plan view illustrating a fundamental method of detecting the target object.
As shown in FIGS. 4A and 4B, the radar device carried in a ship transmits sequentially and alternately a pulse signal PSn (n is a positive integer) for short distance detection and a pulse signal PMn (n is a positive integer) for middle distance detection via an antenna at a predetermined time interval. In this case, the pulse signal PSn for short distance detection is formed of a non-modulated pulse, and the pulse signal PMn for middle distance detection is formed of a modulated pulse.
Here, as shown in FIG. 4B, if a target object 90 with a large reflective cross-sectional area exists in transmitting directions of the pulse signals PM1 and PM2 for middle distance detection, reception signals RE901, 902 corresponding to the pulse signals PM1 and PM2 are saturated at the time of amplification of the reception signals. Therefore, in the signals REC901, 902 after the pulse compression, range side lobes occur as shown in the hatched parts of FIG. 4A. For this reason, problems in which the S/N is deteriorated or reflection signals from other objects are buried in the range side lobes concerned occur to make it impossible to perform an accurate target object detection.
As a method of avoiding the saturation of the reception signal, various kinds of methods exist, which include the followings:
(1) A method of performing amplitude attenuation in analog for the reception signal (i.e., so-called an STC or Sensitivity Time Control processing);
(2) A method of providing a dual system, as disclosed in JPA 2000-137071, of a transmission system which does not attenuate the reception signal and a transmission system which attenuates the reception signal to improve a dynamic range; and
(3) A method of switching between a transmission system which lets the reception signal pass through the LNA and a transmission system which does not let the reception signal pass through the LNA based on detection of the saturation, as disclosed in JPA H09-072955.
However, when the above method (1) is used, a phase changes within one waveform of the reception signal as the amplitude of the reception signal is attenuated. Therefore, if the pulse-compression processing is performed, the range side lobes will occur similar to the case of the above saturation.
When the above method (2) is used, because two or more reception system circuits must be provided, the scale of hardware will be significantly larger. Moreover, an additional dynamic range which can be obtained with such a circuit configuration is small and, thus, the saturation cannot necessarily be controlled sufficiently.
When the above method (3) is used, because the transmission systems are switched therebetween by detecting the saturation, an accurate detection image cannot be obtained for a time period from the detection of saturation to the switching.